Optical Characterization of Scattering Anti-Glare-Layers · Optical Characterization of Scattering...
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1 3/2011 Display-Metrology&Systems © DM&S 2011 Optical Characterization of Scattering Anti-Glare-Layers Michael E. Becker & Jürgen Neumeier Display-Metrology&Systems D-76135 Karlsruhe, Germany
Transcript of Optical Characterization of Scattering Anti-Glare-Layers · Optical Characterization of Scattering...
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Optical Characterizationof Scattering Anti-Glare -Layers
Michael E. Becker & Jürgen NeumeierDisplay-Metrology&Systems D-76135 Karlsruhe, Germany
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Optimization of visual performance of display systems� reduction of disturbing reflections .
� Sparkle� what is it ?� origins of sparkle� visual perception of sparkle
� New approach for measurement and rating of sparkle effects� Measurement setup� Sparkle evaluation
� difference method� spatial filtering
� Results of sparkle evaluationin comparison to visual ratings
� Reduction of unwanted reflections� Characterization of scattering� Evaluation of distinctness of image� Summary
� sparkle� reduction of reflections� distinctness of image
Contents
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Sparkle obtained from a glossy LCD-monitor (plain white image) and a scattering AG-cover glass, a kind of "visual noise".
Intensity, color, and location of sparkle dots change with direction of observation.
Sparkle ?
LCD-screen with AG-glass without AG-glass
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Speckle pattern obtained with laser light transmitted through a scattering
anti-glare treated glass.Department of Physics
University of California, San Diego
Speckle:intensity pattern produced by interference of a set of wavefronts. Intensity varies randomly and as a function of observation conditions.
Speckle ?
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Result of the interaction of light with two structured surface areas:
� the regular display pixel matrix (light source) and � the anti-glare surface with irregular microstructures.
Refraction, diffraction and scattering of light all seem to be involved in the formation of the visual sparkling under discussion here.
Origins of sparkle
Nuijs, Horikx, Applied Optics 33,18(1994)p.4058
Pixel matrix AG surfaces
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On a plain white display:multitude of twinkling dots with different colors and intensities and a random distribution across the display area.
Sparkle pattern: sensitive to the direction of observation (sensitivity more pronounced in the direction perpendicular the long axis of the sub-pixels of the display).
On a monochromatic display:random arrangement of tiny bright and dark dots.
Effect of viewing distance:� visibility of the (sub)-pixels of the display matrix varies, � perception of sparkle does not vary notably.
Binocular observation:dots are most distinct when the eyes are not focused on the display surface.
Monocular observation:small patches (granules) with irregular organic (cell-like) boundaries.
Wedge configuration: staircase pattern of stripes (binocular observation only).
Perception of sparkle
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Measurement and Control of SparkleVisual sparkleon screens with scattering matted anti-glare layers (e.g. touch sensitive panels), � twinkling of the image, � especially distinct when the head of the observer is moving.
Focussing problems and eye-strain claimed by sparkle ...
Effect of display resolutionWith increasing resolution of advanced display screens e.g. for mobile handheld devices (100 - 300 dpi) the problem becomes even more severe.
Visual ErgonomicsReduction of the reflectance of disturbing ambient light sources remains important, since especially mobile devices are often operated under conditions without direct control of the illumination (e.g. outdoors).
Optimization of display systems with respect to visu al performance:� minimization of reflectance of ambient light sources, and� suppression of distinct images of ambient light-sources (fusion conflicts),� AG-layer should not affect the distinctness of the image,� control of disturbing sparkling effects.
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� electronic camera (1300 * 1000 pixels without
color filters), 12 bit digitization,
� magnification in the range of 5 - 25 camera pixels
per display pixel,
� working distance: between 15 cm and 70 cm,
� reproduction of actual application situation:
LCD-monitor with a glossy screen as light-source,
� typically monochromatic / green,
� AG-treated glass sheets on top.
Measurement Setup
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Sparkle Evaluation
Basic problem:separation of the statistic intensity modulations (sparkle) from the regular ones (display pixel matrix).
Two alternative approaches:difference and filter method.
Sparkle: s = σσσσ / µ
σ: standard deviation of intensityµ: mean value of sparkled image
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Image aquisition� record first image,� slightly shift AG-glass laterally,� record second image.
Image of AG-layer on top of display surface
covering half of the region of interest (left half). The right half is the bare LCD-screen.
Regular intensity modulations removed by
image subtraction. Sparkling becomes obvious in the left half of the image.
Difference Method
Evaluation� Subtract one image from the
other. A
G-g
lass
original image difference image
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Single image alternative: may be advantageous, since no mechanical interference (shifting of the AG-layer) is required.
Extraction of sparkling by appropriate spatial filtering, i.e. with masking in Fourier space.
Image of AG-layer on top of display surface covering half of the region of interest (left half). The right half is the bare LCD-screen.
Regular intensity modulations removed by image spatial filtering. Sparkling becomes
obvious in the left half of the image.
Filtering
original image filtered image
AG
-gla
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Comparison of measurement and visual rating of expert observers to check the validity of the measurement method:
� The difference method accurately reproducesthe ranking sequence of the observers.
� The filter method, in some cases, still reverses the ranking sequence.
S p ark le e v alu atio n
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Sparkle values (normalized) fromdifference and filter method for
14 samples compared to (normalized) visual rating of
expert observers (8 categories).
Results
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Avoiding glare and fusion conflicts: � optimized scattering anti-glare layers.
Evaluation of reduction of reflected light intensity: bidirectional reflectance distribution function (BRDF).
Simplified BRDF method integrated in our measurement setup.
Reduction of Disturbing Reflections
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Illumination: linear light source (CCFL) Intensity of reflected light recorded as a function of the location on the display.
Shape of the intensity curve: scattering characteristics of the AG-layer.
Anti-reflection factor AR = max(LAG) / max(LLCD) < 1
Characterization of Scattering
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Image of linear light source reflected in thebare glossy LCD-screen (top) and in an AG-layer (bottom).
Profile of light intensity reflected by the bareLCD-screen (red) and by the scattering AG-
layer (blue).
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Directional variations of the intensity of light reflected from the AG-layers used for sparkle evaluation in this article.
Scattering Characteristics of 14 AG -Glasses
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Micrographs by courtesy of
Dr. Axel Reich, Karsten Schaude
Berliner GlasSurface Technology
Herbert KubatzGmbH&Co. KG
GW1 GW4 GW40
GL70 GL85 GL100
Surface Topographies of AG -Glasses
GW80 GW 100 pixel matrix
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Distinctness of image, DOI: deviation of the direction of light propagation from the regular direction by scattering (in the range ± 0,3°).
� DOI is sensitive to even subtle scattering effects.� DOI is measured to characterize the appearance of e.g.
polished high-gloss surfaces such as automotive car finishes, mirrors.
The scattering of the AG-layer effects a low-pass filtering (blurring).
AGwithoutModulation
AGwithModulationMTF =
Distinctness of Image
unblurred image
blurred image � increasing distance �
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Intensity modulations obtained from the figure to the left. Red: without AG-layer, blue: with AG-layer.
The scattering AG-layer reduces the amplitude and attenuates high-frequency components (smoothing).
Distinctness of Image
AGwithoutModulation
AGwithModulationMTF =
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Requirements:• reduction of reflections from ambient light-sources � low AR-value, • reduction of sparkling effects � low sparkle-value,• maintenance of image quality of the display screen � high MTF-value.
The properties of AG-layers are not naturally relat ed to each other as required for optimum visual display performance.
AG-layer spkl-vis MTF AR GL20 0.0000 0.3088 0.10182 GL30 0.1429 0.4708 0.12581 GL60 0.2857 0.6015 0.31639 GW1 0.2857 0.0189 0.07240 GL75 0.4286 0.7853 0.30988 GL85 0.4286 0.7323 0.37418 GW4 0.5714 0.1303 0.09463 GW125 0.5714 0.9143 0.79860 GW100 0.7143 0.8766 0.70589 GW10 0.8571 0.3312 0.11907 GW80 0.8571 0.8384 0.43105 GW20 1.0000 0.5733 0.17802 GW40 1.0000 0.7124 0.23099 GW60 1.0000 0.7775 0.33958
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Summary of Results
Visual rating of sparkle (spkl-vis), reduction of distinctness of image (MTF) and reduction of unwanted reflections (AR) for 14 AG-treated sheets of glass.
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The proposed method for optical characterization of the sparkle of scattering AG-layers can be used by manufacturers of such coatings for optimization of their products, since the suppression of unwanted reflections and the undesirable reduction of distinctness of image are obtained at the same time.
Manufacturers of electronic equipment that are combining visual displays with touch sensitive input devices can characterize their products for obtaining suitable results concerning suppression of reflections and maintaining distinctness of image.
This metrology method can be a basis for objective rating of theoptical properties and thus for unambiguous communication between customer and supplier.
Sparkle measurement setup available from Display-Metrology&Systems, Karlsruhe, Germany.
Impact
